A.S.
I also have boy/girl twins, and my little girl is the biter. She started biting around a year -- I can't remember exactly when it was. They are now 20 months, and the problem has abated somewhat, though unfortunately it is not over. When she bites, my husband and I give our little girl 1 minute time outs. First, we say very clearly to her, "No biting!" and then put her in her crib for a short time. (If we are not at home, I make her sit in my lap for a minute.) It is hard to tell whether it is working or not, to tell you the truth, but she did stop for awhile and then started again when she was getting her canines. She bites when our son tries to take something from her or when they are both trying to play with the same toy. Lately, she she has been expressing frustration by yelling out angrily if he takes her toy, and we've been praising her for "saying no" instead of biting. I hope it doesn't last too long for you, but most likely it will come and go, as a previous poster mentioned.